The invention relates to angiography, and more particularly relates to method and apparatus used to record angiographic image sequences. In its most immediate sense, the invention relates to lossy JPEG compression of angiographic image sequences such as are generated by, and stored in, angiographic apparatus.
Angiographic apparatus acquires and archives angiographic image sequences, and, advantageously, communicates such image sequences and makes them available over networks. Diagnostic and interventional angiographic procedures produce such vast quantities of data that image compression is required to carry out these functions properly. For example, during embolization of cerebral arteriovenous malfunctions, as much as 2.4 Gb of data may be acquired, at rates exceeding 5 Mb/S. Without compression of image data, the performance of angiographic imaging apparatus would suffer (e.g. storage space would be rapidly depleted and speed of operation would decrease) and the cost of such apparatus would increase. Thus, all angiography apparatus provides for compression of image data. Lossy JPEG compression is one image compression technique that is widely used in such apparatus.
The JPEG algorithm is an image compression algorithm that is defined by the International Organization for Standardization (ISO), the International Telegraph and Telephone Consultative Committee (CCITT) and the International Electrotechnical Commission (IEC). The JPEG algorithm works by dividing up the image into blocks and then transforming each block to the frequency domain using a two-dimensional discrete cosine transform. More specifically, JPEG compression first divides the image into 8 by 8 pixel blocks and then replaces the 64 original pixels with a block of 64 coefficients. The block contains one DC coefficient and 63 AC coefficients; the DC coefficient equals the average value of the 64 pixel block and the 63 AC coefficients express how the image information in the pixel block is distributed in the frequency domain. After the original image has been compressed, it can be reconstructed from the AC and DC coefficients using an inverse discrete cosine transform.
In lossy JPEG image compression, all these coefficients are quantized, i.e. divided by a visibility threshold that degrades their precision. Because the human eye is less sensitive to higher frequencies, the quantization may be coarser at higher frequencies and finer at lower frequencies. By using such a frequency-dependent quantization scheme (known as a quantization table) information that is visually more important is preserved, and information that is visually less important is discarded. It may therefore be understood that selection of the quantization table determines the degree to which image information is compressed, because such selection determines the quantity of information that is discarded.
In lossy JPEG image compression, the image quantization table is generated by multiplying a predetermined table by a scale factor. This scale factor is known as a quantization factor, or Q factor. Consequently, the degree of image compression depends upon the value chosen for the Q factor. The higher the value of the Q factor, the greater the degree of image compression.
Selection of the desired degree of image compression is of great significance to the performance of angiography equipment. If the image compression ratio is too high, important detail may be lost and the medical value of the angiographic image sequences may suffer. If image compression is too low, the equipment will not perform adequately, i.e. will be too slow or will cause overflow and truncation errors. For these reasons, it is advantageous to optimize the degree of image compression, i.e. to so operate the angiography apparatus as to use e.g. an average 12:1 image compression ratio for all incoming image information, and to thereby optimize the tradeoff between image detail and system performance. (It should be noted that the image compression ratio is only meaningful on an average basis; if a sequence of images is compressed on an average 12:1 basis, this means that the entire sequence will be so compressed; the individual images will likely be more or less compressed.)
Unfortunately, such optimization is difficult to do. The JPEG compression algorithm was developed to compress still images rather than image sequences. Different angiographic sequences compress differently even when JPEG-compressed using the same value for the Q factor. In other words, it is impossible to correlate the value of the Q factor and image compression on an a priori basis. This is because the image content of the incoming information is not known in advance. Patient size, organs of interest, collimation setting and acquisition mode (fluoro or cine) all affect this image content. As a result, to arrive at a desired optimal image compression, it is presently necessary to experiment with different values for the Q factor. This is unsatisfactory, because such an approach requires a capable operator and does not permit real time operation.
One object of the invention is to provide angiographic method and apparatus for automatically, i.e. without an operator, determining an appropriate value for the quantization factor Q for a desired average compression ratio C.
Another object of the invention is to provide such method and apparatus that will operate in real time.
Still another object is, in general, to improve on known image compression methods and apparatus of this general type.